The present invention relates to a method and apparatus for measuring degree of vacuum in an electron microscope, and is particularly concerned with a method and apparatus for measuring degree of vacuum in an electron microscope which are preferable for measuring degree of vacuum of a sample part.
In an electron microscope, the degree of vacuum in a body must be measured at all times for efficient monitoring of operation of an electron gun and for obtaining high resolution. A conventional measuring apparatus for degree of vacuum is installed for the purpose of measuring a general degree of vacuum in the body, and as shown in FIG. 7, a measuring apparatus for degree of vacuum 14 is installed at the position of a pipe drawn out of a body 1, thereby measuring a general degree of vacuum in the body. A reference numeral 13 represents a vacuum pump.
A working principle of the conventional measuring apparatus for degree of vacuum is shown in FIG. 8 (reference being made to "Vacuum Art", Lectures on Experimental Physics 4, Kyoritsu Shuppan). The conventional apparatus comprises an anode 15, a magnet 16, two cathodes 17, a high voltage supply 18, and a microbeam ammeter 19. A volume of thermal electrons is generated on a high voltage impressed by the high voltage supply 18. The thermal electrons collide with a neutral gas (gas before ionized) in a space between the two cathodes 17, and a further volume of thermal electrons is generated thereby. The thermal electrons are closed in the space between the two cathodes 17 by a potential arising from the anode 15 and a field arising from the magnet 16. The neutral gas incident externally is ionized by the thermal electrons closed in as above. Since a mass of the ionized gas is sufficiently heavy as compared with the thermal electrons, it reaches the cathodes without being influenced by the potential and magnetic field in the space between the two cathodes 17. Consequently, a degree of vacuum can be decided by measuring an ionic current on the microbeam ammeter. Relation between a degree of vacuum P and an ionic current I may be given by: EQU I=cP.sup.k (k=1 to 2) (c being constant)
The conventional measuring apparatus for degree of vacuum is characteristic of requiring the high voltage supply 18 for generating the thermal electrons and the magnet 16 for closing in the thermal electrons.
Then, the measuring apparatus of this kind comes in those which are disclosed in Japanese Patent Laid-Open No. 66330/1982, Japanese Patent Laid-Open No. 66331/1982 and others.
Meanwhile, it has been considered that a deterioration of degree of vacuum is caused mainly by the gas discharged from a sample specimen. Accordingly, it is conceivable that a measuring apparatus for degree of vacuum be utilized positively as a means for obtaining information on a surface condition of the sample specimen in line with observation on an electron microscope. However, a general degree of vacuum in the body only is measured, as mentioned above, on a conventional electron microscope, and hence there has been no such case where the degree of vacuum of a sample specimen part or thereabout is measured directly. That is, in a measurement of the degree of vacuum on the conventional measuring apparatus for the electron microscope, the situation is such that a pressure of the gas discharged from all parts in the body is measured, but the degree of vacuum only of the sample specimen part is not measured precisely. Further, the degree of vacuum varies according to area as an exhaust resistance depends on structure within the body, or particularly on diameter and length of the piping. Accordingly, the problem is that a correction must be performed in consideration of the structure within the body before obtaining a degree of vacuum of the sample specimen part precisely.
Then, in case the conventional measuring apparatus shown in FIG. 8 is installed in the body of an electron microscope, the high voltage supply 18 for generating thermal electrons and the magnet 16 for closing in the thermal electrons must be provided, therefore an extra high voltage and magnetic field are applied in the body. Particularly, the magnetic field for closing in the thermal electrons must have a strength almost as strong as a magnetic field of the electron microscope body, and thus it is forecasted that a performance of the electron microscope may sharply drop to cause disturbance of an image, deterioration of a resolution and the like from installing the conventional measuring apparatus for degree of vacuum in the body.
Then, for the requirement of discharge prevention, there is a limit for narrowing the space between the anode 15 and the cathodes 17, thus involving a difficulty of miniaturization of the apparatus as a whole. Consequently, the conventional measuring apparatus for degree of vacuum cannot be installed on the sample specimen part within the body, and hence a degree of vacuum of the sample specimen part or thereabout cannot be measured directly.